This invention relates to signal detection circuits and more particularly to circuit means which automatically determines pulse position or arrival time in a way that is independent of amplitude variations and minimizes the effects of unwanted noise on the detection circuits.
It is often necessary in nuclear spectroscopy, radar and communications systems to determine the relative time positions of a sequence of received pulses. If the pulse positions are nearly periodic or exhibit a simple recursion relation, then the conventional time discrimination methods of radar systems can be effectively employed. If not, then different techniques known as time position methods must be employed. If the shape and duration of the pulses are known, then established methods using a matched filter or methods based upon the estimation of "centroid" are used. However, if the signal structure is incompletely known, these methods will not be satisfactory.
An additional problem that arises when detecting received pulses is that they are usually accompanied by impulse and thermal noise. The former is usually caused by electrical disturbances generated externally or by neighboring systems and the latter by the primary stages of the receiving system. In pulse time systems where transmitted intelligence is desired from the timing of a pulse edge, noise may displace the pulse edge from the value corresponding to the signal. Noise impulses in effect may modulate other characteristics of the signal pulse such as amplitude, and slope of the pulse edges and width of the pulse but the ultimate result is translated into a pulse time displacement, producing a distorted intelligence signal. While there are many types of interfering noise pulses, only those that actually coincide in time with the signal edge will create any difficulty in determing the time of receipt of such edge. It is therefore desirable to eliminate, as much as possible, the effects of such noise on the detector circuit.
In the past, two methods have been used most frequently to determine the true time of receipt of an incoming signal. Both systems utilize only the slope characteristics of the incoming signal to determine its time of receipt. One such method is described in U.S. Pat. No. 3,766,411 to Arnold. This device is a peak detector which provided a signal output every time the input signal crosses zero, or whenever it has a slope of zero. While Arnold's method circumvents having an output signal which is dependent upon the maximum amplitude of the input signal, substantial noise remains which may falsely trigger the detector circuit.
The other means of timed-pulsed detection is disclosed in U.S. Pat. No. 2,975,367 to Adams and Harvey. This circuit utilizes the maximum slope of an input signal pulse to provide a time of receipt output pulse. This circuit utilizes a delay line between the input signal and output pulse indication of its receipt and is undesirable in many applications because it causes signal degradation. In addition, the means of Adams and Harvey is costly and does not provide maximum protection from false triggering due to noise.
It is therefore an object of this invention to determine the relative time position of a received signal independent of its maximum amplitude.
Another object of this invention is to determine the relative time position of a received signal without the use of a delay line.
A still further object of this invention is to determine the relative time position of a received signal by a method which minimizes the effect of signal noise.
Yet another object of this invention is to determine the relative time position of a received signal while obtaining a maximum amount of false alarm protection.